CHROMIC-ACID ANODIZING OF ALUMINUM

Chromic acid anodizing of aluminum has been studied by high-resolution scanning electron microscopy and energy dispersive X-ray microanalysis. In the initial stages of anodizing, aluminum oxide protrusions form on the outer surface similar to those formed on etching in either mixed chromic-sulfuric acid or in the FPL etch solution. On nonplanar surfaces the protrusions form preferentially on ridges and merge laterally to form a continuous deposit along the ridges. When the barrier layer has grown to its voltage-determined thickness, the rate of ejection of aluminum ions into solution falls, the growth of protrusions stops, the porous layer develops and dissolution of the protrusions and outer surface of the porous layer takes place as anodizing proceeds. High-resolution scanning electron micrographs show clearly the high degree of pore branching and the irregular spatial distribution of pores formed in chromic acid anodizing compared with the more regular and narrower pores formed on anodizing in sulfuric acid. The hardness, abrasion resistance, flexibility and adhesive bonding properties of the anodic oxide formed in chromic acid are related to the observed structure.